[Guide] <---I need a more comprehensive one

I'm reasonably new to the game so I keep running into things I don't understand. Sometimes I'm able to look them up. Sometimes the answers just yield more questions. Sometimes I don't get any answers at all.

I'm playing as a sorceress/wizardess/female wizard.

So here's one that's bugging me at the moment:

Playing on Torment I, I've managed to figure out how to use the dozen Infernal Machines of Regret I obtained from that dancing goat.*) Smashing in door, managed to make the portal, even managed to kill both Leoric and Maghda - eventually, with the Enchantress Feeling Like Chicken Tonight.

I noticed a door in the corner of the room and by the looks of the map there must be a way to get through that door.
1. How do you go through that door? It's locked. I can't open it. I tried smashing it in, but that didn't help. I picked up all items from the monsters I killed and that didn't provide me with a key.
2. What's behind the door?

These questions also apply to the Enchantress Quest I did when I played the game for the first time, on Normal level. At the end there's also a bit locked off. Couldn't break through it.

Help is as always very much appreciated. When I'm not about to get killed I'm taking screenshots and hope to add to a proper guide once I figured out what it is I'm doing.





*) Where are the drumming goats when you need one?

 

Not sure about that particular door but there could be a different way in. A smashable wall or something else. That's what is usually the issue behind non breakable doors.

 

Thanks....so why do I have to portal out of it and isn't there a portal stone or indeed a portal door? Not really expecting anyone to know though it would be nice if someone knew.

One question leads to the next.

Moved on to act II - I think I know where I can find another infernal machine, but what do I do with that, more specifically, where? Also in New Tristram in the Healer's Hut? If so - how do I get there? I've not figured out how to travel between acts as you do in good old Diablo II. And when - need to complete act II first I imagine?

So many questions....that's even before the one I jotted down on Act V when I played that in normal mode which reads "what's the cloud - colour of the insides of fireworks - that appears when you step into those marshes?"